1. Field of the Invention
The present invention relates to a current-voltage conversion circuit, and more particularly to a current-voltage conversion circuit with a limiting function which is used for example in optical communications or the like. This application is a counterpart application of Japanese application Serial Number 180763/2002, filed Jun. 21, 2002, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
Current-voltage conversion circuits contain signal amplifiers used in various applications. A limiting amplifier which is used for example in the relays of optical communications systems or the like is known as one type of signal amplifier. Known amplifiers of this type include (for example) the limiting amplifier disclosed by Murayama et al. in “10 Gb/s Low Power Limiting Amplifier for Optical Communication Systems” (Electronics Society Conference of the Electronic Information and Communications Society (Denshi Joho Tsushin Gakkai) 1999, C-10-24).
FIG. 7 is a circuit diagram which shows the essential construction of the limiting amplifier disclosed in the abovementioned reference.
In the limiting amplifier 700 shown in FIG. 7, the voltage signal is input into an input terminal 702 via a capacitor 701. The voltage signal that is input into the input terminal 702 will hereafter be referred to as the input signal. The input signal passes through inverters 703, 704 and 705 used for amplification, and is output from an output terminal 709. The logical level of the input signal approaches the logical levels set in the inverters 703, 704 and 705 used for amplification as a result of passing through these inverters. For example, inverters with a DCFL (direct coupled FET logic) constructed from MESFET (metal semiconductor field effect transistors) made of GaAs are used as the amplifying inverters 703, 704 and 705; the structure of these inverters will be described later.
In the case of such a limiting amplifier 700, in order to amplify the input signal with a high degree of precision, it is desirable that the voltage value that constitutes the center of the amplitude of the input signal and the logical threshold values of the inverters 703 through 705 be caused to coincide. The reason for this is as follows: specifically, if the voltage value constituting the center of the amplitude of the input signal and the logical threshold values of the inverters 703 through 705 do not coincide, the ratio of the high level of the input signal will be higher than the ratio of the low level in cases where the voltage value that constitutes the center of the amplitude of the input signal is higher than the logical threshold values of the inverters 703 through 705, and the ratio of the high level of the input signal will be lower than the ratio of the low level in cases where the voltage value that constitutes the center of the amplitude of the input signal is lower than the logical threshold values of the inverters 703 through 705. As a result, amplification will be performed on an input signal in which the ratios of the high level and low level are non-uniform (non-uniform duty ratio).
Accordingly, in the limiting amplifier 700 shown in FIG. 7, a negative feedback circuit consisting of resistance elements 706 and 708 and a capacitor 707 is installed in order to cause the voltage value constituting the center of the amplitude of the input signal and the logical threshold values of the inverters 703 through 705 to coincide. The resistance element 706 and capacitor 707 form an integrating circuit, and produce the mean value of the output voltage of the inverter 705. This mean value is superimposed on the input signal of the inverter 703 via the resistance element 708. Accordingly, in the case of the limiting amplifier shown in FIG. 7, the voltage value that constitutes the center of the amplitude of the input signal and the logical threshold values of the inverters 703 through 705 can be caused to coincide with an extremely high degree of precision by means of the abovementioned negative feedback circuit, so that (for example) amplification can be performed on an input signal in which the ratios of the high level and low level of the input signal are the same (i. e., an input signal with a duty ratio of 50%). Furthermore, the resistance values of the respective resistance elements range from several tens of ohms to several thousand ohms, and the capacitance values of the respective capacitors range from several hundred pF to several hundred nF.
FIG. 8 shows the construction of a current-voltage conversion circuit equipped with a limiting function that uses such a limiting amplifier 700. FIG. 8 is a circuit diagram which shows the construction of this current-voltage conversion circuit. The current-voltage conversion circuit 800 equipped with a limiting function shown in FIG. 8 has a construction in which the current signal is input from the input terminal 702 without passing through a capacitor 701. This current-voltage conversion circuit 800 utilizes the voltage drop of the resistance element 708 to convert the current signal into a voltage signal. Accordingly, the product of the amplitude of the current signal and the resistance value of the resistance element 708 is equal to the amplitude of the voltage signal that is input into the inverter 703.
In the current-voltage conversion circuit 800, it is necessary to increase the amplitude of the voltage signal that is input into the inverter 703 in order to obtain a high sensitivity. Accordingly, in the current-voltage conversion circuit 800, in order to obtain a large signal as the input signal, the amplitude of the voltage signal that is input into the inverter 703 is increased by increasing the resistance value of the resistance element 708.
However, in cases where the inverters 703 through 705 of the current-voltage conversion circuit 800 are constructed using a DCFL structure of MESFETs made of GaAs, the following drawback arises: namely, if the amplitude of the input signal is increased, distortion occurs in the waveform of the voltage signal that is input into the inverter 703.
The term “distortion” in this case refers to a phenomenon in which the rate by which the voltage increases relative to the rate by which the current increases is reduced in the current-voltage characteristics (in other words, a phenomenon in which the slope of the current-voltage characteristic curve is reduced). Such distortion is generated on the basis of the following principle: specifically, in the current-voltage conversion characteristics of the negative feedback circuit, since the resistance value of the resistance element 708 within the negative feedback circuit is fixed, a Schottky current is generated as the potential of the voltage signal reaches a high level exceeding the Schottky barrier of the DCFL, and this Schottky current flows into the gate of the inverter 703. In this case, the voltage signal that is input into the inverter 703 is interfered with by the Schottky current. As a result, distortion is generated in the waveform of the voltage signal that is input into the inverter 703.